Process for preparing chromium film products



Aug. 31, 1965 A. J. DEYRUP 3,203,876

PROCESS FOR PREPARING CHROMIUM FILM PRODUCTS Original Filed Aug. 9, 19612 Sheets-Sheet l INVENTOR ALDEN J. DEYRUP ATTOR NE Y Aug. 31, 1965 A. J.DEYRUP 3,203,376

PROCESS FOR PREPARING CHROMIUM FILM PRODUCTS Original Filed Aug. 9. 19612 Sheets-Sheet 2 INVENTOR ALDEN J- DEYRUP BY Q2 0. @444 ATTORNEY UnitedStates Patent sssass Claims. c1. 204-43 This application is a divisionalapplication of my c0- pending application Serial No. 130,317, filedAugust 9, 1961, which is a continuation-in-part application of myapplication Serial No. 844,906, filed October 7, 1959 which is nowabandoned.

This invention relates to processes for producing chromium films andproducts comprising such films.

In the metallizing of many objects, of metal, cloth, paper, Wood,cement, masonry materials and others, a very desirable means is toadhere with an adhesive to them a thin, self-sustaining film containingthereon a very thin layer of metal, of the order of /2 to 100microinches. A microinch is one millionth of an inch. A thin metal covercan economically confer a brilliant metallic appearance; repel heat byreflecting radiation; conserve heat by the low emissivitycharacteristics of bright metallic surfaces; protect outdoors theunderlying materials such as polymers, cloth, etc. from the damagingeffects of ultraviolet by reflection of the same, as well as minimizingthe transmission of water vapor. Although metal foils can be used forthe same purposes in the same way, the extremely thin metal layeradhered to a selfsustaining film possesses the advantages of greaterflexibility without visible damage to the metal layer, and relativefreedom from wrinkling resulting in better appearance. Also, it is moreeconomical to produce and use because less metal is required.

Metallized polymer films with nickel, copper, and especially aluminumhave been used for this purpose. However, their applications have beenseriously restricted by the easy corrosion of these metals, which isexceptionally damaging at the desirable low thicknesses of /2 to 100microinches. For example, vacuum deposited aluminum on polyester film iswidely used in shredded form in textiles to impart glitter. Alkalinedetergents cannot be used on such fabrics because they dissolve thealuminum. The above metals, when used thus to metallize objects,generally require a lacquer coating for protection. Even so, they arenot suitable for outdoor use because of the damaging effect of light onthe lacquer, and the corroding action of the outdoor atmosphere on themetal.

A composite film consisting of a thin self-sustaining film and, bondedthereto, a thin film of pure, highly refiective chromium would be veryuseful for this purpose, because chromium is extremely resistant tocorrodants including all those commonly present in outdoor exposure.Attempts have been made to deposit chromium on polymer film by pyrolysisof chromium carbonyl. However, deposits made thus are known to be veryhard and brittle and to be composed not of chromium metal but rather ofchromium oxide mixed with chromium carbide. Attempts have been made alsoto metallize polymer film by vacuum vapor deposition, but such depositshave been low in reflectance, i.e., a reflectance of less than 60% ofthe reflectance of a polished 302 stainless steel, as measured by aHunter Reflectometer, and the deposited metal is poorly adherent. It isknown that vacuum vapor-deposited chromium on organic polymericmaterials does not have a bright metallic surface and, therefore, it isnot used as a bright metal film.

3,203,876 Patented Aug. 31, 1965 By the term pure chromium as usedthroughout the specification and claims is meant chromium of at leastpurity as distinguished from chromium oxide or carbide, obtained bypyrolysis of chromium carbonyl.

It is, therefore, an object of this invention to produce transferablepure bright chromium films having a thickness of between /2 andmicroinches and preferably between /2 and 20 microinches.

It is another object of this invention to produce ex ceedingly thin purebright chromium films on an adhesive supporting film.

It is still another object of this invention to produce exceedingly thintransferable pure bright chromium films by electrodeposition on ametallic surface.

It is another object to provide a method of transferring exceedinglythin electroformed pure bright chromium films from a metal surface ontosubstrates of different kinds.

It is yet another object of this invention to provide both flexible andrigid base materials having bonded thereto by means of an adhesive anexceedingly thin layer of pure bright chromium.

Other objects of this invention will become apparent from the followingdescription when read in connection with the accompanying illustrationsin which:

FIGURE 1 is a diagrammatic perspective view showing electroformedchromium and nickel layers on a chromium, stainless steel or chromiumalloy surface and a method for removing said combined chromium andnickel layers with an adhesive film;

FIGURE 2 is a diagrammatic side elevational view of an embodiment ofapparatus suitable for the continuous production of electroformedchromium transfer film;

FIGURE 3 is a drawing of the X-ray diffraction pattern of chromiumproduced by the conventional commercial hexavalent chromium platingprocess; and

FIGURE 4 is a drawing of the X-ray diffraction pat tern of chromiumproduced by a divalent or trivalent chromium plating process.

It is known that bright continuous metal foils, aflixed by adhesives topolymer films, can be prepared by electroforming a thin layer of metal,such as nickel or copper, upon a temporary metal base, applying anadhesive laye and a relatively thin strong polymer film layer, curingthe adhesive, and pulling the composite metal film-adhesivepolymer filmaway from the temporary base. However, it is necessary that thetemporary metal base have very special properties to provide highadherence to the electrodeposited film to hold it during formation, yetlow enough adherence so that the electrodeposited film may be strippedoff when desired. Polished stainless steel or other chromium alloy andchromium plated metal surfaces have these special characteristics, whenthe composition and pH of the plating bath to be used with them aresuitably adjusted.

Attempts to prepare similar bright continuous metal films by thistechnique have been totally unsuccessful. Bright chromium, plated bywhatever means and whatever electrolytic conditions, adheres sotenaciously to stainless steel or previously deposited chromium platethat it cannot be detached without destruction.

Attempts have also been made to first deposit a metal film, for examplenickel, which it is known can be detached, on the temporary base; thenplate bright chromium thereon by the conventional commercial hexavalentchromium plating process; then apply an adhesive layer andself-sustaining polymer film layer; then cure the adhesive; then detachthe composite. Such attempts have also been met with entire failure.When conditions of .pH and composition of the nickel plating bath arechosen suitably for detachment of nickel alone, then during the chromiumplating step the nickel and the chromium both simultaneously detachthemselves, for reasons unknown, breaking up into tiny curl flakes. Whenconditions are progressively altered so the nickel layer is held morefirmly to the temporary stainless steel or chromium base layer is nopoint where adherence during the chromium plating step becomes adequateuntil the bond of the nickel to the temporary base becomes so strongthat nothing can be detached. I have, however, discovered, in accordancewith this invention, that very excellent composite films composed of athin, self-sustaining strong polymer, a chromium layer and a layer ofeither nickel, copper or iron can be obtained if, instead of using acommercial hexavalent chromium bath, one uses a trivalent or divalentchromium plating bath, for example, a chromium plating bath as describedand claimed in Deyrup U.S:P. 3,006,823, issuul October 31, 1961; DeyrupU.S.P. 3,009,333, issued December v18, 1962, and Berzins U.S.P.3,021,267, issued February 13, 1962. With any of these baths, asatisfactory tolerance of conditions can be determined for the priorelectrodeposition step, whet-her it be nickel, copper or iron isimmaterial, where the composite metal layer will adhere during thechromium deposition step, yet come off by stripping with an adheredself-sustaining film when desired. The divalent baths of Deyrup U.S.P.3,009,333 are especially to be preferred because the chromium films arethe brightest.

Exceedingly thin electroformed chromium film may be produced inaccordance with this invention by electroplating a thin layer of nickel,copper or iron (between 3 and 60 microinches in thickness) on a smoothsurface of chromium containing metal, e.g., chromium, stainless steel orother chromium alloy, electroplating chromium, from a bath in which thechromium is divalentor trivalent, to a thickness of /2 to 100microinches over said nickel, copper or iron electroplate, adhering aself-supporting flexible film to the surface of said chromiumelectroplate, and with the self-supporting film, pulling the combinedunderlying metal and chromium electroplates from the chromium-containingmetal support surf-ace. The nickel, copper or iron electrodeposit maythen be readily removed by dissolving the same in a liquid that willattack such metal but is inert to chromium.

While the nickel, copper or iron layer appears bright initially, I findit necessary or desirable to remove it so that pure bright chromiumremains, because these metals tarnish or even corrode badly, in contrastto chromium, resulting in undesirable appearance and poorheat-reflective quality. It is also undesirable to leave on theunderlying electrodeposited metal even when the chromium layer is ontop, and the metal is sandwiched between the chromium and the adhesivelayer. 'In this case, the underlying metal, being of relatively poorcorrosion resistance, can be attacked by weather exposure, resulting inloss of bond and detachment of the chromium layer.

The resulting, exceedingly thin, non-selfsupporting pure bright chromiumfilm substantiallyfree from other metals and backed with aself-supporting film of organic matter, is a unique product that hasmany ornamental and utilitarian uses. It has been found that such thinpure bright chromium film, which is highly resistant to corrosion, maybe tightly and permanently adhered to substantially any kind of surfacefor surface protection and ornamentation. It can be permanently bondedto metals, for example, copper, aluminum, iron or carbon steel toprotect the surface thereof against corrosion and greatly enhance itsappearance. It can be permanently bonded to paper, fabrics, ceramicobjects, transite sheeting or concrete.

The chromium films of this invention may be produced 'batchwise orcontinuously, as illustrated in the drawings. Referring to theembodiment shown in FIGURE 1, reference numeral designates a supportingbase plate having a metallic chromium-containing surface layer 12. Thebase plate may be composed of any metal or even any other rigidelectrically conductive material; however, the

4 surface layer on which the electroplate is to be formed must becomposed of a chromium-containing metal, fo example, chromium, stainlesssteel, or other chromium alloy the chromium content of which is about 8%or more, since it serves as a parting layer, i.e., a layer from whichthe electroplated nickel and chromium film can be stripped. The baseplate 10 may conveniently be formed of polished brass on which thechromium layer 12 is formed by chromium electroplating. In thedescription of the drawings, specific reference shall be made to the useof nickel as the underlying base electrodeposit. Although nickel ispreferred because more work has been done therewith, it is to beunderstood that copper or iron electrodeposit can be substituted withlike results for the nickel.

The nickel layer 14 is electroplated over the chromiumcontaining metallayer 12 and pure chromium layer 16 is electroplated over the nickellayer 14. It has been found that nickel, copper or iron can be plated ona chromium-containing metal surface with just the desirable amount ofadhesion whereby the nickel film will remain adhered to thechromium-containing metal surface until it is desired to strip ittherefrom and yet it can be cleanly stripped by an adhered adhesiveorganic film from this surface when desired, providing that the finallayer of chromium be plated from a bath in which the valence of chromiumis two or three, not six.

A chromium-containing metal surface, i.e., a chromium, stainless steelor other chromium alloy base surface can be replated with nickel andchromium layers and stripped many times without the necessity of bufiingor polishing.

The strippable chromium and nickel electroplates 14 and 16 are so verythin that they cannot support themselves sufliciently to be removed fromthe chromium base layer 12. It is necessary that an adhesive film 22 beadhered to the surface of the top chromium layer 16 and the adhesivefilm combined with the chromium and nickel layers 14 and 16 strippedfrom the chromium, stainless steel, or chromium alloy base layer 12.

The adhesive film 22 may be applied in a liquid form and evened with adoctor knife 20 to form the adhesive layer. After the adhesive layer 22is dry, the composite chromium-nickel-adhesive film can be stripped fromthe chromium, stainless steel, .or other chromium alloy base. In theevent that the adhesive is of such a nature as not to form aself-sustaining film, a paper, cellophane or other film can bepositioned on the adhesive to become a part of the composite strippedfilm. Alternatively, the organic stripping film may consist of paper orcellophane containing a layer of pressure-sensitive adhesive.

The stripped composite film will have the nickel, copper, or iron filmon the outside and chromium film between the adhesive film and the othermetal film. The nickel, copper, or iron film can be easily removed toexpose the chromium surface by dissolving the underlying metal in achemical that will be inert to the chromium, for example, a 10% solutionof FeCl -6I-I O.

A continuous film of pure bright electro-formed chromium on an adhesivesupporting film can be produced on a continuous chromium surfaced,stainless steel or other chromium alloy metal belt as shown in FIGURE 2.In accordance with the embodiment of apparatus shown in FIGURE 2,continuous metal band or belt 60, having a chromium-containing metalsurface, is passed from guide rolls 58 and 46 over plating roller 38which rotates in a nickel plating electrolyte in tank 30 where theexposed outer surface of the belt is electroplated with nickel in aconventional manner. The nickel electroplated belt is then passed overguide roller 48 and washing roller 40 in tank 32 where any remainingelectrolyte is washed from the belt. The belt is then led over guideroller 50 and chromium plating roller 42 which rotates in a chromiumplating electrolyte, containing chromium in a divalent or trivalentform, in tank 34 where the belt is electroplated with chromium over thenickel electroplate. The belt is then subjected to a second washing stepby passing the same over guide roller 52 and washing roller 44 in tank36. The washing solutions in wash tanks 32 and 36 are preferably cleanwater but may contain small quantities of alkaline detergent toneutralize any acid electrolyte that may be carried over from theplating baths.

The belt is then passed over guide rollers 54 and 56 to an adhesiveapplying area. An adhesive that will form a relatively strongself-sustaining supporting film by solidification from a liquid isapplied in any desired manner, for example, by spraying a film-formingsolution of polyvinyl alcohol from a nozzle 62 and smoothed out with adoctor blade 64, preferably raised about 0.08 to 0.5 mm. from thesurface of the chromium plate. After thorough solidification andadherence of the supporting film to the chromium plated surface of thebelt, it is passed over stripping rollers 66 and 68. The thin compositefilm 70, composed of supporting layer, chromium layer and nickel layer,is stripped from the chromium plated, stainless steel, or otherchromium-containing metal surface of the belt 60.

The film 70 is passed from stripping roll 68 over guide roll 72 to guideroll 82. Between rolls 72 and 82 the film 70, with nickel plated surfaceat the top is treated to remove the nickel plate. This may beaccomplished by spraying the film with a solvent as from nozzle 74, thenwashed with water as from nozzle 76 and dried, for example, by blowingwarm drying air against both sides of the film which now consists ofonly the organic film and pure bright chromium film. As illustrated, thedrying air is passed against the film from blowers 78 and 80. From guideroll 82 the chromium-organic film is wound on a mandrel to form roll 84.

As above mentioned, the nickel plate can be made to have just the rightdegree of adherence to the chromium plated, stainless steel, or otherchromium alloy base surface. The nickel plate may be varied to between 3and 60 microinches. The lower limit is chosen so the chromium-nickelcomposite layer will not be too difficult to remove. The upper limit ischosen to avoid an unnecessarily heavy nickel layer to be laterdissolved away. In general, adherence decreases moderately withincreasing nickel thickness. Small amount of copper in the plating bathwill also lessen nickel adherence.

A variation in the process of nickel plating by the well known nickelsulfate-nickel chloride plating process (Watts bath) that will decidedlyinfluence the degree of adhesion of the nickel plate to the chromium,stainless steel, or other chromium alloy base surface is a variation ofthe pH value of the bath. Nickel plating from a Watts bath is usuallycarried out at an acidity between pH 2.0 and pH 5.2. The adherence ofthe nickel plate to the chromium or alloy at this acidity is relativelylow to very low. On the other hand, when plating from such a bath at ahigh acidity, i.e. at a pH value of less than about 1.6, the adherencebecomes relatively high. Generally, it is preferred to plate from aWatts bath or a modified Watts bath at a pH between 1.6 and 2.

Increasing the current density at which the plating takes place willalso generally increase the adherence of the plate. t was, furthermore,found that preconnection of the chromium or alloy base plating surfaceto the power supply before introduction into the nickel plating bath hasa profound effect in decreasing adherence of the plate.

After completion of plating some increase of adherence may be obtainedby heat treatment, for example, by heating in air or with boiling water.It is evident that the factors governing adherence of a nickel plate ona chromium or chromium alloy surface are many and they areinter-related. An optimum adherence for one plate may vary considerablyfrom the desired optimum in another plate. Also a variation in adhesiveorganic film may require a different adherence. An adjustment of theplating process in accordance with the above-described details willpermit adjustment to the desired adhesion by an operator of the process.By far the most important variable to govern adhesion is the control ofthe pH of the nickel plating bath. If the other conditions aremaintained substantially constant the adhesion can be completelycontrolled by the adjustment of the pH of the bath within theabove-described limits of 1.6 to 2.

While nickel is most suitable for the purpose of providing the thindetachable metallic support for the chromium layer, it is also practicalto use either copper or iron for this purpose. For example, the chromiumor chromium alloy base may be coated by electrolysis with copper from asolution containing 1 mole per liter of copper sulfate, the pH beingadjusted within the range 0.2 to 4 by addition of sulfuric acid. Or thesaid base may be coated by electrolysis with iron from a solutioncontaining 1.6 mole per liter of ferrous chloride, and 3.1 moles perliter of calcium chloride, together with sufficient hydrochloric acid toadjust the pH in the range of 0.2 to 4. In either case, the samethicknesses are most useful as in the case of nickel, namely 3 to 60microinches. Also, in either case the pH should be adjusted, dependingsomewhat on the other conditions of electrolysis such as temperature andcurrent density, so that the adherence of the copper or nickel will besufficient to retain the same attached to the base while the chromium isbeing deposited, and yet low enough so that the composite metallic filmscan readily be detached as necessary in the later step of the process.

In the event that a new chromium-containing metal base surface is to beprepared, the metallic base for the chromium-containing metal surfacewhether of the batch type of FIGURE 1 or the continuous type of FIGURE2, care should be taken to clean the metallic base surface, for examplebrass surface, well to secure uniform coating of chromium-containingmetal thereon. There are many known processes for the thorough cleaningof surfaces to be plated and such steps are no particular part of thisinvention. A brass-surfaced base plating structure may be cleaned, forexample, by immersing for a few seconds in a 10% sodium phosphatesolution (10% in water) at room temperature and scrubbed over the entiresurface with a paper towel wet with the same solution. After rinsingwith tap water it is immersed in hydrochloric acid for 510 seconds.After a second rinsing these treatments are repeated. After a finalrinsing the panel preferably drains without a water break. The degree ofcleaning may be altered to suit the necessities of the processor.

The following examples are given to illustrate certain preferredprocedures to produce the chromium-self-sustaining films of thisinvention, it being understood that the invention is not to be taken aslimited to these details.

Any nickel, copper, or iron plating process may be used for the presentinvention. Suitable plating processes are exemplified as follows:

The nickel plating solution is prepared by dissolving 120.0 gm. NiSO -7HO, 24.0 gm. NiCl -6H O, 15.2 gm. H 30 in water and making up to 400 ml.by addition of water containing 0.05% by weight of Duponol ME (thesodium salt of lauryl alcohol sulfate). The pH value of the solution isthen adjusted by adding hydrochloric acid to a pH of between 1.6 and 2.0(preferably about 1.94:.02). If it is found that the adherence of thenickel plate is too great, raise the pH, or if too low, lower the pHslightly.

This solution is stable and may be used repeatedly.

The chromium plate over the nickel, copper, or iron plate may be appliedfrom any one of many chromium plating baths in which the chromium ispresent in the divalent or trivalent state. In order that the combinedmetal-chromium film be plated at a relatively high speed and can beremoved without apparent tearing, it is essential that this chromiumplate have a thickness of not more than about 100 microinches. There isno lower limit in this respect on the thinness of chromium plate whichmay be used. However, at thicknesses below /2 microinch the refiectancesof light and heat by the chromium film and also the pleasing brilliantappearance decrease very rapidly. Several of these chromium platingprocesses will be hereinafter described, it being understood that theyare independent inventions described and claimed in other patentapplications and are no part of this invention.

In plating from the divalent chromium plating bath, described below, thenickel, copper, or iron plated base surface is placed in a chromiumplating solution made by mixing gm. Na CrE; and 0.30 ml. of n-octylalcohol in 29.0 ml. of a stock solution composed of 40 ml. 70% glycolicacid, 40 ml. sodium glycolate solution (made by mixing 35.0 gm. 70%glycolic acid, 24.0 gm. NaHCO and 50 ml. water); 12 gm. sodium formateand 200 ml. of

1 mole per 1000 gm. 4NaF-5B O solution are added to complete the stocksolution.

The compound 4NaF-5B O and its preparation is described in US. Patent toClare and Deyrup No. 2,823,095. The compound Na CrF may be prepared inaccordance with the process described in U.S. Patent t-o Deyrup No.2,996,353, issued August 15, 1961.

The plating is carried out with the bath at a temperature of about C.with graphite anodes, and with a current density of about '70 amperesper square foot. Plating for a period of 2 seconds to 10 minutes willproduce plates of 0.5 to 150 microinches.

The plating may, alternatively, be carried out with a trivalent chromiumplating bath. Such a bath may be prepared by mixing 350 ml. of 98%formic acid in 2000 ml. water and then at a temperature of about 100 C.reacting this dilute formic acid solution with 180 gm. CrO dissolved in500 ml. water. The formic acid solution is placed in a 6-liter flaskfitted with a reflux condenser and a magnetic stirrer. The chromic acidsolution is placed in a dropping funnel. A few drops of CrO solution areadded to the formic acid solution and the latter is then stirred andheated to boiling. \Vhen the reduction of CrO by HCOOH starts (change ofcolor from red to blue-green), the C-rO solution is added slowly fromthe dropping funnel. Since the reduction of CrO by HCOOH is accompaniedby evolution of heat and CO in large quantities this reaction can beviolent when too concentrated reactants are used or if the CrO solutionis added too fast. After the addition of CrO the chromic formatesolution is kept at boiling temperature until the reduction of CrO iscompleted.

Three hundred ml. of 4 molar glycolic acid and 380 ml. of 4 molar soduimglycolate solution are added to the hot chromic formate solution.Glycolic acid will quickly reduce traces of CrO left in the formatesolution.

Two hundred seventy gm. of sodium formate is then added to the hotsolution.

Eighty-three gm. NaF and 310 gm. H are mixed and dissolved in 1000 ml.of boiling water. Small amounts of insoluble material (sometimes presentin NaF) are filtered off, and this solution added to the bath.

The bath is then diluted to 6 liters by adding water and is chilled toroom temperature. The pH of the bath is preferably maintained between3.9 and 4.1.

Plating with this bath may be carried out with the bath at about roomtemperature, using graphite anodes and a current density of about 70amperes per square foot. This process plates with high efficiency, aplate of 0.5 to 100 microinches being produced in from 2 seconds to 10minutes plating time.

Another suitable chromium electroplating bath, the chromium being in thetrivalent state, was prepared in the following manner: A solution of 348gm. of 70 weight percent glycolic acid in 500 ml. of Water was heated to90 C. To this was slowly and cautiously added a solution of 200 gm.chromic anhydride (CrO in 400 ml. of water, while maintainingtemperature at 90100 C. After this addition was completed, the mixturewas boiled for one-half hour. Then 22 gm. of 70% glycolic acid was addedand the solution was boiled down to 800 ml. to ensure complete reductionof hexavalent chromium to trivalent. To this solution was added 435 gm.potassium sulfamate; 38 gm. sulfamic acid; 120 gm. boric acid; 1 gm. ofn-octyl alcohol; and water to a final volume of 2000 ml. The pH wasadjusted to 2.3 by small additions of potassium hydroxide and/ orsulfamic acid. This bath was used at room temperature, with currentdensities of about amperes per square foot, depositing pure brightchromium at a current eflicieucy of about 8%.

As another example of a suitable bath, in which the chromium is in thedivalent state, and which provides an exceptionally strong bright purechromium film, the following solution and specific conditions of use aregiven. A mixture was made of 0.57 gm. mole of chromium (II) chloride;1.17 gm.-moles of sodium formate; 1.33 gm. moles of boric acid; 2.90gm.-moles of sodium chloride; and water to a total weight of 100 gm.This solution was made with materials substantially free from sulfur andselenium (less than 10 p.p.rn.). To the solution was added 0.15% byweight of Petrowet R, a saturated hydrocarbon sodium sulfonate, and 0.1%by weight of n-octyl alcohol, to improve uniformity of distribution ofelectrodeposits. This solution was found to deposit exceptionally strongchromium film when used at 55 C. and current densities of 100450 amperesper square foot, the current efiiciency increasing with current density.

While all of the above represent practical conditions which will lead tobright chromium films having a purity in excess of Cr in the metallicstate, the baths containing divalent chromium are preferred because theyconsistently produce the brightest chromium.

As above stated, after the nickel and chromium plating, aself-sustaining adhesive strip, having sutficient strength to pull theplate from the base surface, will be adhered to the outer chromiumsurface and the composite nickel-chromium-adhesive film will be pulledor stripped from the chromium, stainless steel, or other chromium alloybase surface. Adhesives of substantially all classes may be used toadhere the chromium plate to an adhesive strip. The adhesive mayconstitute the stripping film itself or the adhesive may be of suchnature that a separate fibrous or nonfibrous organic film is necessaryto be adhered to the plated film.

The flexible, self-sustaining films used to strip the metals from thechromium-containing metal base surface must, of course, be sufficientlystrong to withstand the necessary pull without breaking. In order toproduce particularly desirable composite films, the properties of theadhesive film may be important. It is preferred that the adhesive filmhave sutficient hardness and toughness that the stretch of the film beas low as possible to avoid objectionable splitting, cracking or tearingof the plate by the stripping operation. It is also preferred to use afilm having a thickness between about .05 and 5 mils. A number ofparticularly desirable adhesive films will be hereinafter discussed, itbeing understood that the scope of the invention includes adhesiveflexible film or strips in general regardless of the material of thefilm or strip or the adhesive used therewith.

Polyvinyl alcohol films have been used to good effect as strippingfilms. The polyvinyl alcohol can be applied from a 9% solution in water.Since most polyvinyl alcohol films are water soluble, the subsequenttreatment to remove the nickel plate will have to be done with a solventother than with an aqueous solvent. Polyvinyl alcohol film may, however,be treated with an agent to render the same water insoluble in a knownmanner, for example, by treating the same with a cross-linking agentsuch as glutaraldehyde.

Epoxide resin films such as glycidyl polyethers of dihydric phenols maybe obtained by reacting a poly-functional phenol such as resorcinol orbisphenol (2,2-bis(4- hydroxyphenyl)propane) with an excess ofepichlorohydrin or glycidyl polyethers of polyhydric alcohols may beobtained by reacting a polyhydric alcohol, such as glycerol, with anepoxy ether, such as bis(2,3-epoxypropyl)ether. Such films may be veryrapidly solidified by contacting the same with a diamine such asethylene diamine. The ethylene diamine may be liquid or gaseous thusmaking the use of such epoxy resins particularly useful in a continuousplating process as illustrated in FIGURE 2 of the drawings herein.

Another particularly useful film is the acrylic resin film produced froma combination of an acrylic interpolymer and a phenol, urea ofunreamelamine formaldehyde resin. Such resin composition may becompounded as follows:

Percent, by weight Aqueous dispersion of interpolymer A (35.6%

solids) 81.4 Aqueous dispersion of water dilutable, heat reactivephenol/formaldehyde resin (33% so ids) 4.6 Aqueous ammonium hydroxide(28% NH (to pH value of about 9.4) 1.8 Water 12.2

Total nonvolatile, percent 30.5

The interpolymer A was prepared by emulsion polymerization from thefollowing polymerization charge:

The polymerization reaction was carried out under an atmosphere of aninert gas, such as, e.g., nitrogen, in a suitable polymerization vesselequipped with stirrer, re flux column, thermometer and inert gas inlet.The water was deoxygenated by refluxing for about 15 minutes under theatmosphere of nitrogen and cooled to about 150 F. before adding thepolymerizable monomers. The dispersing agent was added to thedeoxygenated water along with the sodium bisulfite, after which themixture of polymerizable monomers was added followed by addition ofpotassium persulfate as an aqueous solution. The aqueous polymerizationcharge was maintained at about 140 F. for about 2 hours or until thepolymerization reaction was complete. Although the preferredpolymerization temperature is about 140 F., the polymerization may becarried out at room temperature or as high as the refluxing temperatureof the polymerization charge. After the polymerization reaction had runto completion the heated aqueous interpolymer dispersions were freed ofresidual monomers by blowing with air, which also removed some of theWater thereby concentrating the dispersion.

The water-dilutable phenol/formaldehyde resin above referred to iscommercially available as Bakelite BR15100 at 66% solids in an aqueousmedium. The ratio of phenol/formaldehyde resin to the interpolymer on adry basis is 5:95.

There are a number of ways whereby the electroformed metal film can betransferred with an adhesive to the final substrate. The simplest toconsider is direct transfer. Here the adhesive is applied to the metalfilm while it is still on the base support surface. Alternatively, theadhesive may be applied to the final substrate. Then the final substrateis pressed to the forming base. When these are separated, the metal filmshould be di- 10 rectly transferred to the final substrate. Usually,while the substrate and base support surface are pressed together, theassembly is heated and cooled, to soften or cure and then set theadhesive layer. This method can best be practiced withnickel-chromium-organic composite film produced batchwise as illustratedby FIGURE 1.

For example, nickel-chromium composite plating on a chromium basesurface was sized with a layer of polyvinyl acetate emulsion55% solids,using a doctorblade to establish reasonably uniform thickness. This wasdried, and then covered with a piece of canvas. These were placed in aCarver press, pressured to about 1200 p.s.i., then heated to 93 C., held5 minutes, Water cooled, then reheated to 40 C. The pressure wasreleased, and the canvas was immediately separated from the chromiumbase. It carried with it the entire Cr-Ni composite. This was etchedwith aqueous 10% FeCl -6H O by flowing this on the metallized surface.This removed the nickel. The product showed a bright smooth chromiumsurface, with very little textile pattern texture. Reheating in an ovento C. for 5 minutes relaxed the structure so that it remained bright,but the textile pattern reappeared.

Equally good results are obtained by sizing either the compositenickel-chromium plating or the substrate to which it is to be applied,in those cases where the.sub strate is relatively nonporous, such assteel or Wood.

Another principal method for transfer is to pick the metal plating offthe chromium or chromium alloy base support with a flexible stripbearing a pressure-sensitive adhesive, and then size the exposed metalwith a different adhesive, and then press this to the final substrate,and then detach the flexible strip with the pressure-sensitive adhesive.This latter could presumably be reused, just as the chromium basesupport is reusable. It will be evident that this turns the metal layerupside down twice. Thus, if chromium is plated over nickel, and thistransfer process is used, the final position on the substrate will benickel side down next to the adhesive. Since this situation is notgenerally desired, it is expedient to etch the nickel off the chromiumwhile both are on the flexible strip with the pressure-sensitiveadhesive.

Cellophane tape containing a pressure-sensitive adhesive, Scotch Tape,was applied to nickel-chromium plating and the composite film strippedfrom the support, thereby placing the Cr-Ni on the adhesive with thenickel side up. This was moistened with 10% aqueous FeCl -6H O andrinsed after the nickel was dissolved. The chromium surface was sizedwith polyvinyl acetate emulsion and placed on a piece of cotton cloth.The assemblage was clamped and allowed to dry. When the Scotch Tape waspulled off, it separated cleanly with the pressure-sensitive coatingstill tacky, leaving the bright chromium transferred to the cloth.

A nickel-chromium composite plate, similarly transferred to Scotch Tape,was sized and air-dried. After drying, the emulsion side was placed on apiece of bond paper. The assembly was clamped between asbestos sheetsand heated in an oven at 1l5-120 C. for 20 minutes. After cooling, theassemblage was pulled apart. The metal film was firmly attached to thepaper.

It is believed that the best mode for transfer of the Ni-Cr compositefilm is to adhere the same to a self-supporting organic film and stripthe same from the base support to form a self-supporting film having anextremely thin chromium surface. This composite film may then beattached to any desired base material.

Particularly desirable results may be obtained by mounting the chromiumfilm of this invention on films composed of a high-melting, difiicultlysoluble, microcrystalline, cold-drawing, linear, highly polymerizedester of terephthalic acid and a glycol of the series HO CH CH In mostcases it is desirable to remove the nickel plate immediately after thecomposite film is stripped from the chromium supporting base. In somecases, if the nickel is allowed to remain on the chromium plate themetallic film becomes dull or frosty in appearance. Microscopicexamination shows that the metal film has been broken in a myriad offragments which have curled with the chromium on the concave side of thecurl.

The removal of the nickel, or copper, or iron can be easily accomplishedby wetting the nickel surface with a aqueous solution of FeCl -6H O.This immediately etches the nickel surface which will appear milky orhazy for a period of seconds and then suddenly becomes bright as thelast nickel is dissolved. The FeCl solution containing the dissolvednickel is then washed from the film and the film dried. Although FeCl 6HO has beenfound particularly useful for this purpose, any solvent orsolvent solution that will dissolve nickel, copper, or iron withoutattacking the chromium, or the organic backing film may be used as well.For example, the following may be used for this purpose:

A 10% solution of cupric chloride (CuC1 in water. or a dilute solutionof chlorine gas in water.

To remove nickel, copper, or iron from a composite film in which theadhesive backing consists of polyvinyl alcohol, a water-solublematerial, a methanol solution containing 10% FeCl -6I-I O and 10% CaClmay be used to good effect.

The nickel or other metal is so thin that it will be removed with asolvent within one or two minutes.

The chromium plating to be produced by one of the above-describedmethods may have a thickness of from /2 to 100 microinches andpreferably between /2 and 20 microinches. At thicknesses in the upperend of this range and down to 2 microinches, reflectivity, as measuredwith a Hunter Reflectometer, averaged 84% of that of a highly polished302 stainless steel (an austenitic alloy containing l7-l9% Cr, 8l0% Ni,0.08-0.20 C, 2. Mn max. balance Fe). It should be noted that theappearance of a metallic coating depends on two largely independentproperties. One of these is the degree of microscopic smoothness,related to specular reflection of light as opposed to scatteredreflection of light. The other is total reflectivity, related to totallight reflected as opposed to light absorbed. The chromium coatings ofthis invention are excellent in both respects. By using an etched orfrosted base plate for forming the chromium films of this invention, itwill be evident that the high total light reflectivity will bemaintained, yet a frosted or satin metallic finish can be produced ifdesired.

At thicknesses of below 2 microinches the chromium plate becomesincreasingly transparent with a corresponding decrease in reflectedmetallic appearance. By carefully controlling the thickness, atransparent, and yet metallically reflective film may be produced,useful for light control. Microscopic examination at 1000 of films ofdifferent thicknesses produced in accordance with this invention showhairline cracks; however, such cracks are .not visible to the naked eyeregardless of the thinness of the film.

The composite chromium metal-polymer films prepared as described abovewere examined by various physical and chemical tests. When the metallayer was separated by dissolution of the polymer in organic solvents,itwas found analytically to be pure chromium metal, i.e., having apurity greater than Continuous pieces of the chromium layer showed goodelectric conduct-ion, characteristic of the metallic state. X-raydiffraction examination, however, showed that the chromium metal is inan unusual form, showing none of the well-defined sharp diffractionlines of previously known forms of chromium. This result is consideredto indicate that the metallic chromium is essentially amorphous, or ofextremely small crystallite size, or that the crystal lattice is highlydistorted. This form of metal is unique in its combinations of thisproperty together with high metallic reflectance, of over 75% of that ofhighly polished 302 stainless steel. Although chromium deposited onmetals such as heavy nickel from the usual commercial hexavalent bathsalso has high reflectance, it cannot be subjected to detachment by theprocess herein described. X-ray diffraction patterns of chromiumdeposited from commercial hexavalent baths show the familiar sharppattern of crystalline chromium shown in FIGURE 3 of the drawings. Incontrast, the X-ray diffraction pattern of the chromium metal describedabove, deposited by the process of this invention, shows the diffuse orwashed-out effect of FIG- URE 4, thus indicating a lack of crystallinityor crystals of an exceedingly small size as compared with the size ofcrystals of chromium deposited from a hexavalent chromium plating bath.

The chromium plate films of this invention are highly resistant tocorrosion as is evidenced by the following tests:

Exposure to saturated water vapor containing 0.5% sulfur dioxideproduced no effect during six weeks exposure. In contrast, steelarticles plated with chromium over nickel by commercial chromium platingprocesses are badly corroded and pitted in one to three days.

Throughout the specification and claim-s, any reference to parts,proportions and percentages refers to parts, proportions and percentagesby weight unless otherwise specified.

Since it is obvious that many changes-and modifications can be made inthe above-described details without departing from the nature and spiritof the invention, it is to be understood that the invention is not to belimited to said details except as set forth in the appended claims.

I claim:

1. The process which comprises electroplating a metal taken from thegroup consisting of nickel, copper and iron to a thickness of about 3 to60 micro-inches on a smooth chromium-containing metal base surface,electroplating chromium, from a chromium salt taken from the classconsisting of divalent and trivalent chromium salts, to a thickness of/2 to micro-inches on said metal plate, and removing said twoelectroplated films from said chromium-containing metal base with aself-sustaining flexible adhesive film adhered to said chromium plate.

2. The process which comprises electroplating a metal taken from thegroup consisting of nickel, copper and iron to a thickness of about 3 to60 micro-inches on a smooth chromium-containing metal base surface,electroplating chromium, from a chromium salt taken from the classconsisting of divalent and trivalent chromium salts, to a thickness of/2 to 100 micro-inches on said metal plate, and removing said twoelectroplated films from said chromium-containing metal base with aself-sustaining adhesive film adhered to said chromium plate, anddissolving from the surface of said metal-chromium-adhesive film theouter metal surface plate.

3. The process which comprises electroplating nickel to a thickness ofabout 3 to 60 micro-inches on a smooth chromium-containing metal basesurface, electroplating chromium, from a chromium salt taken from theclass consisting of divalent and trivalent chromium salts, to athickness of V2 to 100 micro-inches on said nickel plate, and removingsaid two electroplated films from said chromium-containing metal basewith a self-sustaining adhesive film adhered to said chromium plate, anddis solving from the surface of said nickel-chromium-adhesive film thenickel surface plate.

4. The process which comprises electroplating nickel to a thickness ofabout 3 to 60 micro-inches on a smooth chromium-containing metal basesurface, electroplating chromium, from a chromium salt taken from theclass consisting of divalent and trivalent chromium salts, to athickness of /2 to 100 micro-inches on said nickel plate, adhering athin self-sustaining flexible film to said chromium film, and pullingthe composite nickel-chromiumself-sustaining film from saidchromium-containing metal surface.

5. The process which comprises electroplating nickel to a thickness ofabout 3 to 60 micro-inches on a smooth chromium-containing metal basesurface, electroplating chromium, from a chromium salt taken from theclass consisting of divalent and trivalent chromium salts, to

References Cited by the Exer UNITED STATES PATENTS 198,209 112/ 77Outerbrid'ge 20413 1,731,415 10/29 Grupe 20413 2,105,440 1/38 Miller20413 2,133,685 10/38 Coughlin et al. 204-13 3,069,333 12/62 Deyrup20451 JOHN H. MACK; Primary Examiner.

2. THE PROCESS WHICH COMPRISES ELECTROPLATING A METAL TAKEN FROM THEGROUP CONSISTING OF NICKEL, COPPER AND IRON TO A THICKNESS OF ABOUT 3 TO60 MICRO-INCHES ON A SMOOTH CHROMIUM-CONTAINING METAL BASE SURFACE,ELECTROPLATING CHROMIUM, FROM A CHROMIUM SALT TAKEN FROM THE CLASSCONSISTING OF DIVALENT AND TRIVALENT CHROMIUM SALTS, TO A THICKNESS OF1/2 TO 100 MICRO-INCHES ON SAID METAL PLATE, AND REMOVING SAID TWOELECTROPLATED FILMS FROM SAID CHROMIUM-CONTAINING METAL BASE WITH ASELF-SUSTAINING ADHESIVE FILM ADHERED TO SAID CHROMIUM PLATE, ANDDISSOLVING FROM THE SURFACE OF SAID METAL-CHROMIUM-ADHESIVE FILM THEOUTER METAL SURFACE PLATE.